1. Technical Field
The present invention is directed to a lithographic process for device fabrication in which charged particle energy is used to delineate a pattern in an energy sensitive material. The pattern is delineated by projecting the charged particle energy onto a patterned mask, thereby projecting an image of the mask onto the energy sensitive material.
2. Art Background
In device processing, an energy sensitive material, denominated a resist, is coated on a substrate such as a semiconductor wafer (e.g., a silicon wafer), a ferroelectric wafer, an insulating wafer, (e.g. a sapphire wafer), a chromium layer supported by a substrate, or a substrate having a combination of such materials. An image of a pattern is introduced into the resist by subjecting the resist to patterned radiation. The image is then developed to produce a patterned resist using expedients such as a solution-based developer or a plasma etch to remove one of either the exposed portion or the unexposed portion of the resist. The developed pattern is then used in subsequent processing (e.g. as a mask to process (e.g. etch) the underlying layer). The resist is then removed. For many devices, subsequent layers are formed and the process is repeated to form overlying patterns in the device.
In recent years, lithographic processes in which a charged particle beam is used to delineate a pattern in an energy sensitive resist material have been developed. Such processes provide high resolution and high throughput. One such process is the SCALPEL.RTM. (scattering with angular limitation projection electron beam lithography) process. The SCALPEL.RTM. process is described in U.S. Pat. No. 5,260,151 which is hereby incorporated by reference.
Referring to FIG. 1, in the SCALPEL.RTM. process, a mask 10 is used to pattern particle beams 11 and 12. The entire mask 10 is not illuminated at once. Rather, the mask 10 is illuminated in segments (two adjacent segments 25 and 26 are illustrated in FIG. 1). Accordingly, segment 25 is first illuminated by means of particle beam 11 and subsequently segment 26 is illuminated by particle beam 12. Mask 10, as shown, consists of a membrane 13, which is transparent to the particle beams incident thereon.
The developed image of the mask pattern is defined by blocking regions 14, which scatter the particle beams incident thereon. The blocking regions block the particles incident thereon from being transmitted onto the resist-coated wafer 24. In the illustrated example, the mask 10 also has skirt regions 15 on the periphery of the segments 25 and 26. Supporting struts 16 are spaced to define a mask segment 25 and 26. Emerging beams 11a and 12a are that portion of the radiation incident on the mask that is significantly scattered by either the blocking regions 14 or skirt regions 15. Skirt regions 15 are provided because it is preferable for the incident beam to fit entirely within the area defined by the segment and not to contact the support struts. Since the diameter of the beam incident on the mask segment may extend beyond the patterned region of the mask segment in at least one dimension, the skirt regions are provided so that the distance between the struts 16 is greater than the diameter of the beam.
Unblocked illumination, consisting, in sequence, primarily of beams 11a and 12a is caused to converge by means of electromagnetic/electrostatic first projector lens system 17, thereby producing emerging beams 11b and 12b to result in cross-over, e.g. of beams 11c and 12c at position 18, as depicted on the plane of apertured scatter filter 19. Filter 19 is on the back focal plane for the instance in which beams 11a and 11b are parallel to the optical axis.
Second projector lens system 22 is of such configuration and so powered as to bring the beams of each of the bundles (11c and 12c) into an approximately parallel relationship. The action of the lens 22 is sufficient to direct the on-axis bundle 11d into orthogonal incidence onto wafer 24. For the off-axis bundle 12c, redirection is required in order to avoid imaging the struts 14 and skirts 15 that separate mask segments 25 and 26. Such redirection is performed by deflectors 20 and 21. Deflectors 20 are so energized as to redirect off-axis beams such as beams 12c to result in positioned beams 12d. The function of the deflectors 21 is to bring about final directional control so as to result in beams 12e and so as to eliminate images of associated struts and skirts.
Various strategies for blending the images projected from discrete areas of a patterned mask in order to provide a seamless image in the energy sensitive material have been suggested. One such strategy is described in U.S. Pat. No. 5,624,774 to Okino et al. In Okino et al., the mask contains patterned regions separated by border region. The image of a first pattern is transferred into the energy sensitive material. The image of a second pattern is then transferred into the energy sensitive material. Pattern 1 and pattern 2 are separated by a border on the mask. However, the image of the border is not to be transferred into the energy sensitive material, that is, the image of pattern 1 and the image of pattern 2 are to be seamlessly joined (stitched) in the energy sensitive material. In order to accomplish this, Okino et al. have a third pattern on the mask which is an image of the desired seam between patterns one and two (i.e. pattern 3 has a certain portion of the side of pattern 1 adjacent to pattern 2 and comparable portion of pattern two from the side adjacent to pattern 1. Thus Okino et al. have a separate pattern of the desired seam between all of the main patterns on the mask.
As previously noted, the mask used in SCALPEL.RTM. has a plurality of patterned areas separated by struts. In the SCALPEL.RTM. process, it is desired to produce a seamless image of the patterned areas on the mask. If the images of the patterned areas are not precisely joined, then the device that is ultimately fabricated may be defective (e.g., if the pattern is a conductive path and the portions of the image are not properly aligned, conductivity can be reduced or destroyed, rendering the device unuseable). Accordingly, an efficient and accurate process for producing a seamless image from a plurality of discrete patterned portions on a strutted mask is desired.